1. Field of the Invention
The present invention relates to an image processing apparatus that can output copy-forgery-inhibited pattern images, a method for controlling the image processing apparatus, a program, and a storage medium.
2. Description of the Related Art
Special paper called security paper exists. A character string (e.g., COPY) may be embedded in security paper such that the character string is not readily visible at a glance. The embedded character string appears clearly on a copy obtained by copying the security paper. Thus, a document prepared using such security paper can be distinguished from a copy of the document with ease. As a result, a person may hesitate to use a copy of the document.
Security paper has such effects and thus is used when, for example, a resident card or a form is made. However, a problem exists in that the cost of security paper is higher than that of ordinary paper.
In view of higher cost associated with security paper, recently, a new technique has become of interest, in which the same effects as with security paper can be achieved using ordinary paper (see Japanese Patent Laid-Open No. 2005-136953). In this technique, original document data and copy-forgery-inhibited pattern image data created with a computer are combined in a printer, and the combined image data containing copy-forgery-inhibited patterns is output on ordinary paper. Character strings and the like are embedded in this copy-forgery-inhibited pattern image. Thus, the embedded character strings appear clearly on a copy obtained by copying the image containing copy-forgery-inhibited patterns. In this technique, ordinary paper is used. Thus, an advantage exists in that the cost of making an original using this technique is cheaper than the cost of making an original using security paper. Furthermore, in this technique, a new copy-forgery-inhibited pattern image can be created every time an original is made. Thus, this technique has another advantage in that the colors, embedded character strings, and the like of a copy-forgery-inhibited pattern image can be freely set up.
A copy-forgery-inhibited pattern image includes an area that is to remain and the other area that is to disappear (or that is pale compared with the area that is to remain) on a copy of the image. The reflection densities of these two areas on the original are substantially the same. Thus, embedded character strings, for example, “COPY”, are invisible to the human eye. Here, the term “remain” means that an image on an original is correctly reproduced on a copy of the original, and the term “disappear” means that an image on an original is not reproduced on a copy of the original. The reflection density is measured with a reflection densitometer.
Hereinafter, an area that is to remain on a copy is called a latent-image part, and the other area that is to disappear (or that is pale compared with the area that is to remain) from a copy is called a background-image part.
FIG. 11 is an illustration showing the state of dots in a copy-forgery-inhibited pattern image. In FIG. 11, an area where dots are concentrated is a latent-image part, and the other area where dots are dispersed is a background-image part. The dots in these two areas are generated by different types of dot processing, different types of dither processing, or the like. For example, the dots in the latent-image part are generated by dot processing with a small number of lines, and the dots in the background-image part are generated by dot processing with a large number of lines. Alternatively, the dots in the latent-image part may be generated using a clustered-dot ordered dither matrix, and the dots in the background-image part may be generated using a dispersed-dot ordered dither matrix.
The reproduction capability of a copying machine depends on the input resolution and output resolution of the copying machine. Thus, there is a limit to the reproduction capability of a copying machine. Accordingly, in general, when the size of each of dots in the latent-image part of a copy-forgery-inhibited pattern image is larger than the size of a dot that can reproduced by a copying machine and when the size of each of dots in the background-image part of the copy-forgery-inhibited pattern image is smaller than the size of the dot that can reproduced by the copying machine, the dots in the latent-image part are reproduced on a copy, and the dots in the background-image part are hardly reproduced. Consequently, the color of the latent-image part reproduced on the copy is deeper than the color of the background-image part. Hereinafter, the term “visualization” is used to describe a process in which embedded character strings, symbols and the like appear clearly on a copy by reproducing the latent-image part on the copy so that the color of the latent-image part is deeper than the color of the background-image part.
Parts (a) and (b) of FIG. 12 show visualization. FIG. 12 schematically shows that concentrated dots (large dots) are reproduced on a copy, and dispersed dots (small dots) are not correctly reproduced on the copy.
A copy-forgery-inhibited pattern image is not limited to that described above and can take other forms so long as a character string (e.g., COPY) or a symbol appears clearly (is visualized) on a copy so that it can be recognized by a person. Even when a character string (COPY), is white and the area surrounding the character string becomes visible on a copy, the purpose can be achieved. In this case, an area that includes the character string (COPY) is called a background-image part. And, the area surrounding the character string is called a latent-image part.
In the technique disclosed in Japanese Patent Laid-Open No. 2005-136953, when an original document image includes an image that is fully painted with one color, for example, white, a copy-forgery-inhibited pattern image is overlaid on the original document image. This operation prevents a copy-forgery-inhibited pattern image from being concealed by an image that is fully painted with one color, for example, white, and disappearing. When an original document image does not include an image that is fully painted with one color, the original document image is overlaid on a copy-forgery-inhibited pattern image. This operation prevents a copy-forgery-inhibited pattern image from deteriorating the quality of an original document image.
Furthermore, another technique is disclosed in Japanese Patent Laid-Open No. 2005-136953, in which, in addition to determining whether an original document image includes an image that is fully painted with one color, for example, white, it is determined on the basis of the proportion of the image that is fully painted with one color in the original document image whether image compositing is performed so that one of the corresponding images overwrites another image, or image compositing is performed by performing an OR operation (logical disjunction operation) on the images.
In the techniques disclosed in Japanese Patent Laid-Open No. 2005-136953, the most appropriate method for combining a copy-forgery-inhibited pattern image and an original document image is determined, and a composite image is generated. However, it is not assumed that image compositing is performed when a form image exists in addition to a copy-forgery-inhibited pattern image and an original document image. Thus, a method for most appropriately combining these three types of image is desired.